Cable television (CATV) is a form of broadcasting that transmits programs to paying subscribers through a physical land-based infrastructure of coaxial cables or through a combination of fiber-optic and coaxial cables rather than through the airwaves. Thus, CATV networks provide a direct link from a transmission center, such as a headend, to a plurality of subscribers located at typically addressable remote locations, such as homes and businesses.
Cable television networks based on coaxial distribution have been deployed for over half a century. The main function for early cable systems was to provide television service to areas where off-the-air reception was unavailable. In the past thirty years, most cities and county locations have been wired for cable television services. These services have evolved from 2-12 local off-air channels in the 1950s and 1960s to a variety of current services over a signal distribution service transmitting FM radio broadcasts, multi-channel TV programs, pay-per-view-movies (Video on Demand), information services such as videotext, and the like. Many cable systems now originate their own programming in an ever-increasing number of channels. In recent years, novel services have been made available to the subscribers, including interactive services. One such service regards a two-way, interactive communication involving access to established data communication networks, such as the Internet. CATV transmission, however, has been designed mostly to optimize downstream broadcasting; it was not configured for upstream receipt of information from subscribers. Even though upstream transmission has existed for years, recent advances and customer requirements have increased the kind and amount of upstream transmission to such an extent that the infrastructure for transmitting that upstream information has issues needing to be corrected and/or improved.
The signals that are carried over the coaxial cable delivery system are typically received at a headend facility. A CATV headend is the central transmission center operative to gather and to provide complex audio, visual, and data media throughout a geographical area, which can cover most or all of a small city. In big cities or metropolitan areas, multiple headend facilities cover separate areas but can be interconnected redundantly for reliable supply of signals. The signals at the headend are received through, for example, satellite receive antennas, antennas erected on a tower, microwave links, fiber optic cables, and direct coaxial interconnects, and the external signals received through the various types of employed antennas include satellite, microwave, and local TV station broadcasts. Additionally, locally produced and pre-recorded programs can be introduced into the system. The responsibility of the headend is to process and to combine the received signals for distribution to customers and businesses. In addition, the headend assigns a channel frequency to all the signals destined for cable distribution. These single received signals are multiplexed into a group of channels that are spaced 6 MHz apart, which are then offered to the subscribers selectively or are bundled as packages. Pay-per-view and special pay channels are added by keying the subscribers' set-top boxes or by phone authorization from the subscribers. If an upstream channel is operative in the network, the option of electrical authorization can be provided to the subscribers.
Programming has increased from the local off-the-air channels to include local, regional, national, and international programming. More and more channels have been added over the years so that a typical cable system now might offer hundreds of channels with analog and digitally compressed services. Once the signals have been processed at the headend, they can be distributed to the coaxial system through fiber optic cables, microwave transmitters, or directly from the headend over the coaxial network.
A CATV system comprises a plurality of elements, which are operative in maintaining the flow of electrical data information through a coaxial conductor or through a combination of fiber-optic and coaxial cables to subscribers. The infrastructure of the system is required to span vast urban areas by cables installed underground or on high poles. It is routinely expected that the transmitted signals be kept at their highest possible fidelity having the lowest possible random energy interference level and this ability requires the CATV provider to periodically adjust the signals at each interconnect location.
Coupled between the headend and the subscriber end of the CATV system is a system of cables. A plurality of trunk cables, constructed of large diameter coaxial cables or of a combination of coaxial and fiber-optic cables, carry the signals from the headend to a series of distribution points. A typical cable system architecture includes a main trunk cable that is connected between the headend and these distribution points, referred to as hub stations or trunk/bridger stations. One or more feeder cables feed off the trunk/bridger station. Feeder cables branch out from the trunks and are responsible for serving local neighborhoods. Each feeder cable contains a number of taps disposed along the length of the feeder cable, and each tap contains a number of ports. A drop cable is connected between each port and a subscriber end and forms the familiar coaxial cables that enter directly into a CATV subscriber's premises. Terminal equipment is connected to the drop cable inside a CATV subscriber's home through a wall outlet. Among the more common terminal devices are televisions, VCRs, set-top boxes, converters, de-scramblers, cable modems, and splitters. For a system offering two-way communications, the subscriber end also has a terminal that transmits signals upstream, in the return path of the cable system.
FIG. 1 is a schematic drawing of a typical hybrid fiber/coaxial cable-based broadband/CATV telecommunications system. FIG. 1 can represent a typical cable television system that is currently deployed to service cable television subscribers. In the illustrative example shown in FIG. 1, forward CATV signals originate at a headend facility 1 and are supplied to a fiber optic transmitter 2. The fiber optic transmitter 2 transmits the forward CATV signals to a fiber optic node 4 over fiber optic cable 3 (shown with a dashed line). The fiber optic node 4 also transmits reverse path signals from the subscribers to an optical receiver of the headend 1. An optical receiver in or adjacent to the fiber optic transmitter 2 is not illustrated separately but is typically located in the headend 1 to receive and process these return path signals from the optical node 4. The optical node 4 processes the optical signal and can provide a standard RF output signal. The standard RF output signal is then provided to and carried over a coaxial cable 5 (a trunk or main line) to CATV trunk/network amplifiers 6 that are placed (in series) apart from one another with lengths of coaxial cable 5 therebetween. Depending upon the network architecture, the trunk/network amplifiers 6 can supply the signal to a network of distribution cables 9 that feeds signals to a smaller group of amplifiers, typically referred to as distribution or line-extender amplifiers 7. The distribution amplifiers 7 and distribution cable 9 feed passive devices placed near an end user's location to tap off a main signal supply, which devices are sometimes referred to as distribution or subscriber taps 8. The distribution taps 8 supply a signal tap for a subscriber's coaxial cable service drop 10. The subscriber service drop 10 enters a subscriber location 11 and provides the subscriber with desired services, such as television, high-speed Internet, and/or telephone.
It is noted that this embodiment is just one of many different types of CATV distribution architectures and many cable TV operators utilize different devices and equipment to deploy their services to the end subscriber. However, in many cases, systems that utilize coaxial cable to distribute their services deploy a similar architecture of fiber optic cable, coaxial cable, amplifiers, and passive distribution devices.
The signals transmitted from the headend to the subscriber end are contained within a particular frequency band—the forward (or downstream) path (or channel) of the CATV system. The signals transmitted from the subscriber end to the headend, or to some other upstream station, are transmitted in a different frequency band (higher and/or lower) than the forward path frequency band and these upstream transmissions are referred to as the return (or upstream) path (or channel) of the CATV system. When transmitted over fiber optic cables, losses in transmission are much improved and are more stable than when transmitted over coaxial cable. Accordingly, different techniques are required for improving transmission quality. The quality of transmission also is different with respect to the intermediate amplifiers used for fiber optic and coaxial cables.
Coaxial cables are constructed with a center conductor surrounded by a dielectric cross-section and an outer conductor, typically made from an aluminum outer shield. The coaxial cable attenuates the signal in a linear function of its conductor resistance. Different sizes of cable, therefore, attenuate the signal flow at different values due to the size of the center conductor and dielectric material. Booster amplifiers 6, 7 are placed along the coaxial cable. The spacing of the amplifiers 6, 7 along a cable route is determined by the loss of the route and is commonly selected based on the recommended operating gain of the amplifier 6, 7. Typically, the booster amplifiers 6, 7 are located at points where the signal levels have been reduced to a pre-designed level. These amplifiers 6, 7 are designed to add a minimum amount of noise and distortion to the processed signals. But, the amplifiers 6, 7 generate additional noise at various points in their circuitry. A ratio of total input noise power to a thermal noise floor is referred to as a noise figure of a given amplifier. As the amplifiers 6, 7 are not perfectly linear, they also contribute additional distortions each time a signal is amplified. Due to the inherent contributions of noise and distortion (e.g., nonlinearity), the signal can only be amplified a certain number of times before the change in the signal, as compared to the signal provided at the headend 1, becomes unacceptable. The cascade effects of the amplifiers 6, 7 (e.g., net distortion introduce into the signal) typically results in a limited number of amplifiers 6, 7 in a continuous cascade. The limiting factors may include the type of modulation, the total number of channels, and/or a desired performance at the end of the cascade. The Federal Communications Commission (FCC) has developed specific rules and regulations that govern the acceptable minimum performance to a cable customer. In particular, the FCC mandates that all signals provided over a cable system must maintain a peak to valley of less than or equal to less than 10 dBmV for systems of 300 MHz, plus 1 dB for each additional 100 MHz increments or fraction thereof. These rules and regulations must be taken into account during the design process of all cable systems.
One of the characteristics of coaxial cable is that the signal loss is less at lower frequencies (such as at channel 2, for example) than at higher frequencies (e.g., at channel 117). This phenomenon is shown, for example, in FIG. 2. Therefore, the amplifier 6, 7 needs less amplification at lower frequencies than at higher frequencies. One way of describing this correction is that the output of an amplifier 6, 7 is tilted to ensure minimal noise and distortion performance of the downstream signal flow. The output performance of the cable amplifier 6, 7 is typically reduced for the lower channels in relation to the higher channels based on the total number of channels carried on a cable system. The levels into the first gain block of most amplifiers are typically flat, which provides desirable performance. The overall signal levels for all channels must be maintained below a signal level that will not overload the input of a television or other signal reception devices. Because coaxial cable loses more signal as the frequency is increased, the levels of the lower frequencies must be reduced to provide equal power levels of all signals. The signal must be adjusted at the input of a given amplifier to reduce or “equalize” these signals, and circuits referred to as equalizers provide the correction for this transmission loss. The behavior of equalizers is shown in the graph of FIG. 3. The slope or tilt of the amplifier gain is adjusted by installing a fixed value equalizer. These equalizers typically have been available in 1 to 1.5 dB increments. To perform equalization at a particular amplifier 6, 7, a field technician selects proper values to balance that amplifier 6, 7 to a pre-designed output level, stated in dBmV. The result of applying an equalizer is shown in FIG. 4, in which the equalizer response pattern compliments the response pattern of the cable to produce a flat broadband output signal. The amplifiers 6, 7 also have a provision for adjusting forward and reverse gain levels. This is commonly accomplished by the installation of a fixed value attenuator, typically referred to as a “pad.” The behavior of a pad is shown in the example of FIGS. 5 and 6 in which a signal (e.g., of 20 dBmV) is not attenuated in the graph of FIG. 5 and the signal is attenuated (e.g., by a 10 dB pad) in FIG. 6. The pads and equalizers might be installed before the input of the first gain hybrid or at interstage locations that are typically between two gain hybrids. Most legacy and state of the art amplifiers employ fixed cable equalizers. These are commonly plugged into the input or interstage location of the amplifier 6, 7 to reduce power levels of the lower channel.
FIG. 7 is a schematic drawing of a typical standard coaxial amplifier 6, 7 and application of equalizers and pads. Such amplifiers 6, 7 are typically placed at various locations along the trunk and distribution coaxial cables 5, 9. These amplifiers 6, 7 have specific purposes and are placed at pre-designed locations to amplify and equalize the forward and reverse signals. As those skilled in the art will readily understand, such amplifiers 6, 7 vary in design and in a number of output ports to feed different configurations of coaxial cables. Some models feed only one coaxial cable while other may feed many, for example, five different output cables.
FIG. 7 illustrates an example coaxial amplifier 6, 7 with three different forward output and reverse input cables. The forward signal 12 is received through the input coaxial cable 5, 9 and is routed to a RF/AC splitting device 13. In addition, an AC voltage typically supplies an internal power supply, where the AC voltage is converted to a DC voltage to power the amplifier. A bi-directional test point 14 provides the technician a port location to measure the forward input levels (and the reverse output levels as described below). In the example of FIG. 7, an RF portion of the signal is routed to a diplex filter 15. The diplex filter 15 separates and/or combines the forward signals of the cable system. A forward portion of the diplexer 15 is connected to the forward path of the amplifier for amplification. The forward signal flow is routed to an input equalizer 16 and an input pad 17. The input equalizer 16 and the input pad 17 condition the signal to supply a flat input to a first hybrid 18 in the amplifier 6, 7. The input hybrid 18 amplifies the signal by a pre-determined amount. The signal is then processed by another stage, including an interstage equalizer 19 and an interstage pad 20. The interstage equalizer 19 and pad 20 provide a tilted input to a third hybrid 21 in the amplifier 6, 7. This tilted level maximizes performance of the amplifier 6, 7 by reducing distortions and increasing the Signal-to-Noise ratio contributions of each amplifier (there are a total of three third hybrids 21 off the three-way splitter A in this amplifier 6, 7 to form three output ports). In each output channel, the output of the third hybrid 21 is routed to another diplex filter 22, which separates and or combines the forward and reverse path signals for their separate route through the amplifier and cable system. A bi-directional test point 23 provides the technician with a port location to measure the forward output levels (or the reverse input levels). The forward RF signal is, then, combined with the AC voltage at an AC/RF combiner 24 for signal to flow to the output coaxial cable 25 (which can be another coaxial cable 5 or 9 or it can be the subscriber service drop 10).
The reverse path has similar characteristics as the forward path where the coaxial cable exhibits less signal loss at lower frequencies (such as at 5 MHz, for example) than at higher frequencies (42 MHz, for example). The input as measured at the bi-directional test point 23 is balanced to arrive flat into the amplifier to ensure minimal noise and distortion performance of the upstream signal flow. The output performance of the reverse path cable amplifier is typically reduced for the lower frequency in relation to the higher frequencies of the reverse path (typically 30, 40, 42, 55, 65, 85, or 204 MHz). The RF levels into the reverse gain block of most amplifiers are typically flat, which provides desirable performance. The overall signal levels for all frequencies must be maintained at signal levels that will not overload or under drive the reverse hybrid or reverse amplifier. Because coaxial cable loses more signal as the frequency is increased, the levels of the lower frequencies must be reduced to provide equal power levels of all signals. Most amplifiers only have one equalizer location to equalize for the upstream coaxial cable losses. The equalizer is typically installed at the output location of the reverse amplifier to reduce power levels of the lower channel to the upstream amplifier input signal levels.
The reverse signal is received through the coaxial cable 25 and is routed to a RF/AC splitting device 24. In addition, an AC voltage typically supplies an internal power supply, where the AC voltage is converted to a DC voltage to power the amplifier. A bi-directional test point 23 provides the technician a port location to measure the reverse levels. In the example of FIG. 7, an RF portion of the signal is routed to the diplex filter 22. The diplex filter 22 separates and/or combines the forward and reverse signals of the cable system. A reverse portion of the diplex filter 22 connects to the reverse input pad location 20′ on each port. Each port may be padded differently if the design requires different padding levels. These signals are then combined with the additional reverse signals from the two additional ports with a three-way combiner B. The combined reverse signals are then amplified by the reverse hybrid 18′ by a pre-determined amount. The reverse signal is then adjusted with the correct output equalizer 16′ and output pad 17′ to ensure correct balanced levels. The reverse signals are then combined with the forward frequencies at the diplex filter 15 and combined with the AC voltage at an AC/RF combiner 13 for an output signal 12′ to transmit on coaxial cable 5, 9. The bi-directional test point 14 provides the technician with a port location to measure the reverse output levels. The tilted reverse output levels maximize performance of the upstream amplifier by reducing distortions and increasing the signal-to-noise ratio contributions of each amplifier. This ensures a flat signal level across the reverse spectrum to the input of the upstream amplifier.
As the characteristics of coaxial cables and amplifier amplification characteristics vary with temperature changes, equalizer values may need to be changed several times over the course of a yearly time span to reflect the seasonal temperature changes. Each time the equalizer is changed, the cable television signal flow is interrupted while the correct value equalizer is exchanged. (For example, changing the fixed-value equalizer circuits commonly results in a 1-2 minute outage.) The majority of current cable television-based equalizer components are fixed-value plug-in equalizers that are placed in the forward and reverse signal path to equalize the signals on the cable network. One standard configuration for the plug-in equalizers 16, 16′, 19, 26, 29 and the plug-in pads 17, 17′, 20, 20′, 27, 30, 40 is a JXP series plug-in, one of which in the form of a pad 40 is shown in the forward path of optical transmitters 47 and the reverse path of the optical receivers 49 of the optical node 4 of FIG. 9, this optical node 4 being an Opti Max™ OM6000 HFS modular optical node made by Arris.
Fiber optic cable does not have the same shortcomings of coaxial cable. In particular, coaxial cable attenuates the signal in a linear function of its conductor resistance but fiber optic cable does not. Thus, not only can fiber optic cable be laid in much longer lengths from the headend directly to optical nodes (an optical node converts the optical light signal to a standard output signal suitable for a coaxial distribution network) and between and among optical nodes within an optical network (typically configured in a loop to permit transmission from both directions if one optical node fails), equalization between these lengths of fiber optic cable is not needed because fiber optic cable delivers a flat response signal. This results in a transmission circuit where most of the over-land transmission occurs through fiber optic cable with coaxial cable being used primarily only at the subscriber's local area and at the drop to the subscriber's house/business.
Introduction of fiber optic cable has eliminated many of the long coaxial cascades and microwave systems deployed in the 1980's and 1990's. The fiber optic system allows the cable operator to route fiber optic cables closer to a group of customers and supply near headend quality performance at an optical node. The coaxial network then delivers the signal to a customer's home where the customer can receive the transmitted signals with a television receiver, set-top converter box, computer system, and/or telephone receiver.
An optical node typically supplies the broadband communications signal to a group of amplifiers that are capable of amplifying the forward and reverse path signals. A normal format for the signals, for example, is from 54-1000 MHz in the forward path direction and 0-42 MHz in the reverse path direction. Not all coaxial based systems operate at this frequency but most bi-directional systems operate a multi-path concept and can use different forward and reverse bandwidths. These frequencies include but are not limited to 0-40/52-1000 MHz (forward/reverse), 0-42/54-1000 MHz, 0-55/70-1000 MHz, 0-65/85-1000, 0-85/105-1000 MHz, and 0-204/258-1000 MHz. An example of an optical node 4 (in circuit diagram form) with a reverse input pad on each RF port and a single optical pad on the upstream transmitter is shown in FIG. 8 and a picture of an exemplary optical node is shown in FIG. 9. As those skilled in the art will readily understand, such optical nodes 4 vary in design and the number of output ports to feed different configurations of coaxial cables. Some models feed only one coaxial cable 5 while other may feed many, for example, five different output cables 5. The configuration of FIG. 8 illustrates an example of an optical node 4 that feeds output signals 35 to three different output coaxial cables 5. The picture of the exemplary configuration optical node 4 in FIG. 9 feeds four different optical cables. The optical node 4 contains a forward configuration board 41 and a return configuration board 43, each connected to a router board 45. Four fiber receivers 47 and four fiber return transmitters 49 are connected to router board 45. As is typical for virtually all optical nodes 4 on the market presently, each of the receivers 47 and transmitters 49 has a single location for inserting a JXP-style pad 40—in other words, for the transmitters 49, there is no location for an equalizer in the return path and, more particularly, there is no slot present in which an equalizer can be inserted in the return path. A status monitor device is also present at the optical connection side of the optical node 4 (at the bottom of FIG. 9). At the RF connection side of the optical node 4 (the top of FIG. 9), there is an RF module 50 for each of the four cable connections. This RF module 50 has a circuit diagram similar to one of the RF branches of the optical node 4 in FIG. 8. Accordingly, each RF module 50 has two locations in the forward path for inserting a JXP-style pad 17 and a JXP-style equalizer 16. Also, each RF module 50 only has a single location for inserting a JXP-style pad 40—in other words, there is no location for an equalizer in the return path. A power supply 52 and a power distribution board 54 are also present at the RF connection side.
With regard to the FIG. 8 circuit diagram, the forward signal 12 is received via the input fiber optic cable 3 and is routed to an input equalizer 26 and an input pad 27. The input equalizer 26 and the input pad 27 condition the signal to supply a flat input to an input hybrid 28 in the optical node 4. The input hybrid 28 amplifies the signal by a pre-determined amount. The signal is then processed by another stage, including an interstage equalizer 29 and an interstage pad 30. The interstage equalizer 29 and pad 30 provide a tilted input to the third hybrid 31 in the optical node 4. This tilted level maximizes performance of the optical node 4 by reducing distortions and increasing the Signal-to-Noise ratio contributions of each amplifier 31 (and the two other third hybrids 31 off the three-way splitter B). The output of the third hybrid 31 is routed to a diplex filter 32, which separates and or combines the forward and reverse path signals for their separate route through the downstream amplifier and then to the coaxial cable system. A bi-directional test point 33 provides the technician with a port location to measure the forward output levels or the reverse input levels. The forward RF signal is combined with the AC voltage at an AC/RF combiner 34 for signal 35 to flow to the output coaxial cable (which can be coaxial cable 5).
The input reverse signal 35 is received via the coaxial inputs 5. The reverse RF signal is then separated with the AC voltage at the AC/RF combiner 34. The bi-directional test point 33 provides the technician with a port location to measure the reverse input levels. The diplex filter 32 separates and or combines the forward and reverse path signals for their separate route through the node. The three separate reverse signals are routed to respective input reverse pads 40 for attenuation. The signals are combined with a three-way combiner C before amplification. The reverse hybrid 28′ amplifies the signal by a pre-determined amount. The signal is then routed to the reverse optical transmitter 42. Most optical transmitters 42 have a location to install an attenuation pad 40 to ensure the correct drive levels to the optical transmitter input. However, the vast majority of optical nodes do not have an additional location for equalization in the reverse path at the optical node.
Cable operating companies are now offering to their subscribers advanced communication services in the return path, including addressable converter operation, pay-per-view transactions, telephony, interactive digital networks, and computer data transmission. To offer such services in a reliable manner, certain problems in the return path must be addressed. For instance, many CATV cable systems are designed primarily for forward path operation. The loss (or attenuation) values of each tap are selected to provide proper signal levels at the drop cables at forward path frequencies. The forward signal at each successive tap port, ideally, is designed to have the same level at the highest design frequency. This insures a proper forward signal level to each subscriber. It is desirable to selectively control the loss at several points in at least one of the channels of the system. Trunk cables share the same properties, as do generic transmission lines with regard to signal attenuation. But, signals do deteriorate and/or attenuate, for example, as each new tap is added or as an existing tap is removed.
Due to the forward tap design, loss in the return path varies widely with every tap. This causes a corresponding variance in the signal levels in the return path. This variance in signal level imposes severe design constraints on subscriber terminal transmitters (e.g., set-top addressable converters) and adversely affects the ability of headend receivers to properly detect the return path signals. Significant improvements in the return path performance can be achieved by controlling the loss variance in the return path. If the loss at each tap port can be made substantially uniform, the total variance can be brought down to an acceptable level.
As with coaxial amplifiers 6, 7, optical nodes 4 have a provision for adjusting forward and reverse gain levels. The forward slope or tilt of the optical node 4 is adjusted by installing a linear-based equalizer, for example, a JXP-series equalizer, and the forward gain of the optical node 4 is adjusted by installing an attenuator, for example, a JXP-series pad. The forward pads and equalizers are typically installed before the input of the first gain hybrid or at one or more interstage locations that are typically between the gain hybrids.
Like coaxial amplifiers 6, 7, the reverse path signal levels of the optical node 4 are balanced to arrive flat into a reverse optical node station to ensure unity gain from all ports. One common method to balance and adjust the return path is by injecting a pre-established signal level from a Field Service Meter (FSM) at test point 23 of FIG. 7 and at test point 33 of FIG. 8. The balancing signal from the FSM then flows through the upstream amplifiers, through the optical node 4, and then on to the headend 1. The signal level from the FSM is processed at the headend 1 through a specialized headend controller (HEC). The HEC converts the reverse path-balancing signal to a forward path frequency and transmits the information back to the FSM on a dedicated telemetry frequency. The FSM receives the telemetry frequency at test point 23 or test point 33. The specialized channel contains information allowing the technician to adjust for the correct amplitude levels at FIGS. 7 and 8 and install the correct value for the attenuation pads. Reverse equalizers are installed at FIG. 7 in the existing equalizer location but the reverse path cannot be equalized at FIG. 8 because an equalizer location does not exist. This method of unit gain balancing is typically accomplished from the optical node 4 out to the end of the coaxial cable distribution. This ensures that each amplifier in the upstream path has already been balanced correctly, as the technician works toward the end of the line accomplishing the balancing of the forward path and reverse path in one visit. Each amplifier is balanced from the reverse output location to compensate for the tilt developed in the coaxial cable into the next upstream amplifier or optical node. This allows the technician to sweep and balance the forward and return path signals in one visit to each amplifier. The older method of balancing the forward path only and then balancing the reverse path was costly and inefficient. This method of reverse path balancing often-required two technicians, as one technician would measure the incoming signal to the upstream amplifier while the second technician made adjustments on the reverse portion of the downstream amplifier supplying the reverse path signals to the upstream amplifier.
The vast majority or all of optical nodes 4 installed in the U.S. have a location in the output reverse path for an attenuator pad 40 to ensure the flat input levels into the next station upstream, which is accomplished with the installation of the fixed value attenuator/pad at a location for that output reverse path attenuator. However, because the input to the optical node 4 is balanced for flat input levels into the optical node 4 from the downstream amplifier, virtually all optical nodes 4 do not have a location for a reverse equalizer. The performance of the optical link displays a flat response once the input to the optical node transmitter receives a flat input level. This is different from the coaxial cable network, where tilt is developed in the cable transmission. The theory to explain this configuration where there is no equalization in the reverse path of the optical node is that a flat input level to the reverse transmitter 49 in the optical node 2 will deliver a flat output at the optical receiver located in the headend 1. Accordingly, most of the optical nodes 4 installed in the U.S. in recent times do not physically have any location in which a reverse path equalizer can be inserted—in other words, there is no provision for a reverse path equalizer (see, e.g., FIG. 8).
FIG. 10 is a diagrammatic illustration of components at the headend 1. The reverse path signal is received at the headend 1 through an optical receiver 60, which is typically mounted in an equipment rack 61 (which, for example, splits, combines, equalizes, and/or attenuates the signals). The optical signals are converted back to RF and, through coaxial cable 62, supply RF signals to return path equipment (e.g., cable modem termination system (CMTS) 64, VOD 66, set-top box controllers (DAC) 68) inside the headend 1. The cable 62 may span several hundred feet of RG-59 or RG-6 cable. These cables 62 are much smaller in diameter and exhibit more attenuation and tilt-per-foot than the larger, outside, hardline, coaxial cables. Additional coaxial cables 63 are present within the racks 61 to the return path equipment 64, 66, 68 and augment the adverse tilt characteristic. The same tilt characteristics, therefore, occur in the headend cable as described in the outside plant cable, where the coaxial cable exhibits less signal loss at lower frequencies (such as at 5 MHz, for example) than at higher frequencies (42 MHz, for example). Accordingly, the entire communications system requires equalization to arrive flat in the reverse path processing equipment. This equalization of the reverse path signaling has been accomplished at the headend at a very high cost and with very high complexity. In particular, the equalization requires a de-mark location to be installed after the optical receiver 60 inside the headend 1 to process the signals through a separate equalizer circuit, which requires the addition of equipment racks, multiple cables, connectors, and signal management shelves to install the equalizer, which imposes two significantly undesirable costs. First, floor space in each headend is extremely valuable and rare. As such, any requirement for added space is to be avoided. Second, every new rack requires multiple cable terminations, which imparts additional complexity into the system. One commercial example of a headend-resident equalization unit that is costly and requires significant rack space is manufactured by ATX under the trade name MAXNET®.
It would be beneficial to have a system, apparatus, and method to equalize the reverse data signal path to the data network servers from the CATV subscriber where no such equalization presently exists. Adding equalization at the optical node in the reverse path could greatly reduce the reverse path noise contributions often associated with low frequency signal interference. The reverse equalizer has greater attenuation at the unusable lower frequencies from 0 to 12 MHz than at the higher frequencies around 42 MHz (assuming a 42 MHz reverse spectrum). This reduction in the noise can greatly reduce the potential for reverse path laser clipping by reducing the total power input to the laser.
Thus, a need exists to overcome the problems with the prior art systems, designs, and processes as discussed above.